Cables for electric power and for communication lines are run underground in order to protect them from above-ground elements and from the interference and damage they would suffer when installed above the ground or on poles or structures. The underground environment may be less hostile in some ways, but the history of underground cables suggests that the underground environment is not to be considered benign. The environment in underground power and communications manholes is indeed harsh.
While there may be fewer ultraviolet rays and less severe weather underground, and the temperature is more constant, moisture and humidity are always a consideration. There are other considerations, such as the constant and higher danger from flooding, and underground pests that consider electrical insulation, and even steel, a tasty treat. Manholes may fill with water that is often contaminated with sewage, fertilizer runoff, tree roots, and chemicals, including caustic materials. Very harsh sea or salt water sometimes fills manholes. Many manholes are completely or partially filled with such contaminated water all of the time, except when pumped out for maintenance. Others fill periodically but are hot and have extremely high humidity, while still others fill and empty with ocean tides.
Thus, at a minimum, communication and power cables must be kept off the ground, organized, and protected to the greatest extent possible. Cables are thus typically supported underground by racks that elevate cabling and keep the cabling off the ground, thus shielding the cables from at least some of the worst underground dangers. Once the decision is made to go underground, the underground environment must then be faced. Racks for supporting cables must be able to withstand both heat and cold, all conceivable temperatures and humidities in every combination. In addition, the racks must be able to support very heavy and steady loads from power and communication cables. The racks themselves are preferably supported, e.g., attached to a wall, rather than free-standing structures. Thus, the racks will have penetrations, or stress concentrators, to deal with, in these hot, humid, and stressful environments, along with the high loads expected from supporting cabling. The walls themselves will also have penetrations, supporting the bolts or other fasteners used to secure the racks in place. The walls, such as concrete walls or other structures, will also be in intimate contact with the racks, adding their chemical potential for corrosion to the racks.
All these stresses combine to make the underground a challenging environment for cable racks. In the past and still today, many underground cable racks and fasteners are made from hot dipped galvanized steel. The steel arms and posts are bonded together and grounded in an attempt to prevent galvanic corrosion. Galvanic corrosion is caused by a difference in the electrical potential between the galvanized steel and the surroundings, including the manhole and any water in the manhole. Eventually, the galvanized coating is consumed and the steel racks may oxidize or corrode away, leaving the power and communications cables without support.
What is needed are cable racks better able to withstand the environment, and better able to tailor themselves to a greater variety of situations, for fewer stresses, and for longer service.